Exploiting σ/π coordination isomerism to prepare homologous organoalkali metal (Li, Na, K) monomers with identical ligand sets

Tetraamine Me₆TREN has been used as a scaffold support to provide coordinative saturation in the complexes PhCH₂M?Me₆TREN (M=Li, Na, K). The Li derivative displays a Li? C σ interaction with a pyramidalized CH₂ both in the solid state and in solution, and represents the first example of η⁴ coordination of Me₆TREN to lithium. In the sodium derivative, the metal cation slips slightly towards the delocalized π electrons whilst maintaining a partial σ interaction with the CH₂ group. For the potassium case, coordinative saturation successfully yields the first monomeric benzylpotassium complex, in which the anion binds to the metal cation exclusively through its delocalized π system resulting in a planar CH₂ group.     

Surfing π Clouds for Noncovalent Interactions: Arenes versus Alkenes

A comparative study of molecular balances by NMR spectroscopy indicates that noncovalent functional‐group interactions with an arene dominate over those with an alkene, and that a π‐facial intramolecular hydrogen bond from a hydroxy group to an arene is favored by approximately 1.2 kJ mol^?1. The strongest interaction observed in this study was with the cyano group. Analysis of the series of groups CH₂CH₃, CH?CH₂, C?CH, and C?N shows a correlation between conformational free‐energy differences and the calculated charge on the C^α atom of these substituents, which is indicative of the electrostatic nature of their π interactions. Changes in the free‐energy differences of conformers show a linear dependence on the solvent hydrogen bond acceptor parameter β.     

Carbon-13 NMR investigation of 2,5-diaryl-1,4-dithiins

The ¹³C NMR spectra of eight 2,5-diaryl-1,4-dithiins were recorded and signals were assigned. A linear correlation was observed between the electronegativity of the substituent groups on C-10,10′ and the chemical shifts of C-10,10′ after applying corrections for the magnetic anisotropic effect of the substituents. A Hammett correlation was found between the ¹³C chemical shifts of C-3,6 and C-7,7′ and the σ_p⁺ parameter associated with the substituents on C-10,10′. Extended electronic interaction between the π system of the aryl group and the π system of the dithiin ring was suggested by the observance of an alternating behavior in the magnitude of the substituent effects on the ¹³C shifts of C-2,5 and C-3,6. An alternating effect was also noted in the magnitude of the long-range ¹³C? F coupling constants for these same carbon signals in 2,5-(10,10′-difluoro)diphenyl-1,4-dithiin.     

Brønsted酸参与的(SP3)碳－氟键活化：含三氟甲基的芳香化合物分子间或分子内的芳基化反应

三氟甲磺酸被发现能够高效地活化(SP3)碳－氟键。因此,在其催化作用下,室温时含三氟甲基的芳香化合物与苯反应得到二苯甲酮类化合物。在同样的Brønsted 酸催化下,其中一些含三氟甲基的芳香化合物亦可发生分子内的芳基化反应,而此时分子间的芳基化受抑制。强的氢氟间的相互作用或氢键作用,被认为对该Brønsted 酸参与碳氟键活化的反应起了重要作用。     

Carbon Monoxide Bonding With BeO and BeCO3: Surprisingly High CO Stretching Frequency of OCBeCO3

The complexes OCBeCO₃ and COBeCO₃ have been isolated in a low‐temperature neon matrix. The more stable isomer OCBeCO₃ has a very high C? O stretching mode of 2263 cm^?1, which is blue‐shifted by 122 cm^?1 with respect to free CO and 79 cm^?1 higher than in OCBeO. Bonding analysis of the complexes shows that OCBeO has a stronger OC? BeY bond than OCBeCO₃ because it encounters stronger π backdonation. The isomers COBeCO₃ and COBeO exhibit red‐shifted C? O stretching modes with respect to free CO. The inverse change of C? O stretching frequency in OC? BeY and CO? BeY is explained with the reversed polarization of the σ and π bonds in CO.     

Design of new organic superacids with fused and isolated pyrrole and cyclopentadiene rings and assessment of effect of –BX2 (X = H,F, Cl,CN) substituents on the acidity enhancement: A DFT analysis

New classes of organic Brønsted acids were designed with pyrrole and cyclopentadiene scaffolds, and their acidity was assessed theoretically by the B3LYP/6-311++G(d,p) method. The hydrogen atom of NH group in pyrrole was substituted by an –BX₂ (X = H, F, Cl, CN, CF₃). The boron atom stabilizes the conjugated bases by interaction with the center of negative charge after deprotonation. The acidity of the compounds was promoted by substitution of the hydrogen atoms of the rings with CN moiety as a strong electron withdrawing group. Also, after deprotonation, delocalization of the negative charge in both pyrrole and cyclopentadiene rings causes stability of the conjugated bases and consequently enhances the acidity. The charge delocalization in the neutral acids and their conjugated bases was compared using nucleus-independent chemical shift index. Enthalpies and Gibbs free energies of deprotonation in gas phase, ∆H_acid and ∆G_acid, were used as a measure of acidity. Both compounds with isolated and fused pyrrole and cyclopentadiene rings were investigated and it was found that the formers are more acidic. Using these strategies, several acids and superacids with wide range of acidity with ∆G_acid values of 244 to 328 kcal mol⁻¹ were obtained.     

The diiodine basicity scale: toward a general halogen-bond basicity scale

The new diiodine basicity scale pK_BI2 is quasi‐orthogonal to most known Lewis basicity scales (hydrogen‐bond, dative‐bond and cation basicity scales). The diiodine basicity falls in the sequence N>P≈Se>S>I≈O>Br>Cl>F for the iodine‐bond acceptor atomic site and SbO≈NO≈AsO>SeO>PO>SO>C?O>? O? >SO₂ or PS?? S? >C?S?N?C?S for the functionality of oxygen or sulfur bases. Substituent effects are quantified through linear free energy relationships, which allow the calculation of individual complexation constants for each site of polybases and thus the classification of aromatic ethers as carbon π bases and of aromatic amines, thioethers and selenoethers as N, S and Se bases, respectively. The pK_BI2 values of nBu₃N⁺‐N^?C≡N, 2‐aminopyridine and 1,10‐phenanthroline reveal a superbasic nitrile, a hydrogen‐bond‐assisted iodine bond and a two‐centre iodine bond, respectively. The diiodine basicity scale is a general inorganic but family‐dependent organic halogen‐bond basicity scale because organic halogen‐bond donors such as IC≡N and ICF₃ have a stronger electrostatic character than I₂. The family independence can be restored by the addition of an electrostatic parameter, either the experimental pK_BHX hydrogen‐bond basicity scale or the computed minimum electrostatic potential.     

Synthesis and characterization of diorganotin(IV) complexes of Schiff bases with ONO‐type donors and crystal structure of [N‐(2‐hydroxy‐4‐nitrophenyl)‐3‐ethoxysalicylideneiminato]diphenyltin(IV)

A series of neutral complexes, namely, [N‐(2‐hydroxy‐4‐nitrophenyl)‐3‐hydroxysalicylideneiminato]‐ diphenyltin(IV) ( Ia ), [N‐(2‐hydroxy‐4‐nitrophenyl)‐3‐methoxysalicylideneiminato]diphenyltin(IV) ( IIa ) and [N‐(2‐hydroxy‐4‐nitrophenyl)‐3‐ethoxysalicylideneiminato]diphenyltin(IV) ( IIIa ) were prepared by the reaction of diphenyltin dichloride on the corresponding Schiff bases. The Schiff bases were the reaction products of 2‐hydroxy‐4‐nitroaniline and appropriate salicylaldehydes. All the compounds were characterized by elemental analysis, ¹H‐NMR, ¹³C‐NMR, IR and mass spectroscopy. Compound IIIa was also characterized by single crystal X‐ray diffraction and shows a C₂NO₂ coordination geometry nearly half‐way between a trigonal bipyramidal and square pyramidal arrangement. In the solid state, π? π interactions exist between the aniline fragments of neighbouring molecules. Copyright © 2007 John Wiley & Sons, Ltd.     

Can Conventional Bases and Unsaturated Hydrocarbons Be Converted into Gas‐Phase Superacids That Are Stronger than Most of the Known Oxyacids? The Role of Beryllium Bonds

The association of BeX₂ (X: H, F, Cl) derivatives with azoles leads to a dramatic increase of their intrinsic acidity. Hence, whereas 1H‐tetrazole can be considered as a typical N base in the gas phase, the complex 1H‐tetrazole–BeCl₂ is predicted to be, through the use of high‐level G4 ab initio calculations, a nitrogen acid stronger than perchloric acid. This acidity enhancement is due to a more favorable stabilization of the deprotonated species after the beryllium bond is formed, because the deprotonated anion is a much better electron donor than the neutral species. Consequently, this is a general phenomenon that should be observed for any Lewis base, including those in which the basic site is a hydroxy group, an amino group, a carbonyl group, an aromatic N atom, a second‐row atom, or the π system of unsaturated hydrocarbons. The consequence is that typical bases like aniline or formamide lead to BeX₂ complexes that are stronger acids than phosphoric or chloric acids. Similarly, water, methanol, and SH₂ become stronger acids than sulfuric acid, pyridine becomes a C acid almost as strong as acetic acid, and unsaturated hydrocarbons such as ethylene and acetylene become acids as strong as nitric and sulfuric acids, respectively.     

1,2,4‐Triazol‐3‐ylidenes with an N‐2,4‐Dinitrophenyl Substituent as Strongly π‐Accepting N‐Heterocyclic Carbenes

The synthesis and characterisation of a series of new Rh and Au complexes bearing 1,2,4‐triazol‐3‐ylidenes with a N‐2,4‐dinitrophenyl (N‐DNP) substituent are described. IR, NMR, single‐crystal X‐ray diffraction and computational analyses of the Rh complexes revealed that the N‐heterocyclic carbenes (NHCs) behaved as strong π acceptors and weak σ donors. In particular, a natural bond orbital (NBO) analysis revealed that the contributions of the Rh→C_carbene π backbonding interaction energies (ΔE_bb) to the bond dissociation energies (BDE) of the Rh? C_carbene bond for [RhCl(NHC)(cod)] (cod=1,5‐cyclooctadiene) reached up to 63 %. The Au complex exhibited superior catalytic activity in the intermolecular hydroalkoxylation of cyclohexene with 2‐methoxyethanol. The NBO analysis suggested that the high catalytic activity of the Au^I complex resulted from the enhanced π acidity of the Au atom.     

Chiral Recognition in π Complexes of Alkenes,Aldehydes, and Ketones with Transition Metal Lewis Acids; Development of a General Model for Enantioface Binding Selectivities

Prochiral alkenes, aldehydes, and ketones constitute the most frequently used starting materials for enantioselective organic syntheses. Protocols often involve chiral binding agents or Lewis acids that can give two diastereomeric adducts, the ratios of which are measures of chiral recognition. With π adducts, the diastereomers differ in the enantioface of the C?C or O?C group bound to the Lewis acid. This review provides the first comprehensive analysis of such equilibria and related binding phenomena with chiral transition metal Lewis acids. An extensive body of data from the authors' laboratory for complexes of the pyramidal rhenium fragment [(η⁵?C₅H₅)Re(No)(PPh₃)]⁺ ( I ) affords particular insight. Literature data for other complexes are also summarized. A general model for chiral recognition based upon the relative steric properties of four quadrants is presented. This enables binding selectivities to be individually and rationally optimized for different classes of ligands. Electronic effects are also identified and correlated with specific structural properties. Relationships between binding equilibria, reactivity, and product configurations are discussed.     

Study of the nature of the interaction of CH3CN with Lewis acids

The unperturbed ν⁰ (CN) stretching frequencies of CH₃CN in 18 complexes with Lewis acids are correlated with ν (CN) frequencies in cobalticyanides and with cation parameters which measure σ and π interactions. The results indicate that the nature of the interaction is predominantly of the σ donor—acceptor type wib no detectable π back-donation from the cation to the CN ligand.     

Syntheses,weak interactions,and 3-D network structures of two salts based on [Ni(i-mnt)2]2− (i-mnt2−=iso-maleonitriled-ithiolate) anion with substituted pyridinium cation

Two new salts, [2-NaMePy]₂[Ni(i-mnt)₂] (1) and [2-NaMe-4-MePy]₂[Ni(i-mnt)₂] (2) ([2-NaMePy]⁺ = 1-(2′-naphthylmethyl)pyridinium, [2-NaMe-4-MePy]⁺ = 1-(2′-naphthylmethyl)-4-methylpyridinium and i-mnt^2? = iso-maleonitriledithiolate), have been prepared and characterized by elemental analyses, UV, IR, molar conductivity, and single crystal X-ray diffraction. The anions in 1 form a 1-D chain through short C ··· N interactions between the anions, while the cations in 2 stack a 1-D column via C–H ··· π and π ··· π stacking interactions between the cations. The effect of weak intramolecular interactions such as C–H ··· N, C–H ··· S, C–H ··· Ni hydrogen bonds, and π ··· π stacking interactions between the cations and the anions further generate a 3-D network structure. The change of the molecular topology of the countercation when the 4-substituted group in the pyridine ring is changed from H atom to CH₃ group results in different crystal system, space group, and the stacking mode of the cations and anions of 1 and 2.     

A “double-zeta” type wavefunction for an organometallic: Bis-(π-allyl)nickel

A wavefunction which is of double-zeta quality at the level of the valence orbitals [based on a (11, 7, 5/8, 4/4) gaussian basis set contracted to (4, 3, 2/3, 2/2)] is reported for thebis-(π-allyl)nickel molecule. Independant SCF calculations for two ionized states substantiate the conclusion reached previously for a number of organometallics with a minimal basis set that Koopmans' theorem is not valid for these molecules, namely that the highest occupied orbital from the ground state calculation for the neutral molecule is mostly a ligand π orbital whereas the lowest ionization potential corresponds to the removal of an electron from a molecular orbital which is mostly a metal 3d orbital. The nature of the bonding inbis-(π-allyl)nickel is discussed on the basis of the possible interactions between the metal orbitals and the π orbitals of the allyl group. The interaction between the filled nonbonding π orbital of the allyl group and the empty 3d _xz orbital of the Ni atom appears responsible for most of the bonding, together with some backbonding through an interaction between the 3d x ²?y ²and 3d xyorbitals and the σ and π orbitals of the ligands. The computed value for the rotation barrier about the C-C allyl bond, 90 kcal/mole, rules out this rotation as one of the possible mechanisms which account for the equivalence of the terminal hydrogens in the proton magnetic resonance spectra of π-allyl complexes.     

Recherches sur la formation et la transformation des esters LXVII. Note sur la scission des acides ω-(N-phénylthiocarbamylamino)-alcoyl-sulfuriques à divers pH

In aqueous solutions at 100°C varying from HCl 2N to NaOH 2N , N-phenylthiocarbamoyl derivatives of aminoalkylsulfuric monoesters C₆H₅NH? CS? R? OSO₃H are split in the following ways:

• (a) With R = ? CH₂? CH(CH₃)? or ? (CH₂)₃? the scission of the monoester group is very rapid in the hole pH-range studied, especially in alkaline medium; the resulting cyclic products, 5-methyl-2-phenylamino-thiazoline and 2-phenylimino-tetrahydrothiazine respectively, formed by nucleophilic attack of an unshared pair of the S atom on the C bearing the monoester group, have been isolated and identified.
• (b) With R = ? (CH₂)₄? , the rate of the scission in alkaline or neutral medium is very much higher than that of an alkylsulfuric monoester; in these media a cyclic product is also formed (this time by nucleophilic attack of the unshared pair of the thiocarbamoylated N atom on the C bearing the monoester function) which has been isolated after alcaline scission, and identified as N-phenylcarbomoyl-pyrrolidine. In acid medium, no special influence of the phenylthiocarbamoyl group is observed.
• (c) With R = ? (CH₂)₅? or ? (CH₂)₆? , the rate of the scission in alkaline medium is 30 to 1000 times lower than in the previous cases; no pure organic scission products have been isolated. In acid or neutral medium, these two esters behave like usual alkylsulfuric acids.

     

Mechanistic Insights into the Ni‐Catalyzed Reductive Carboxylation of C−O Bonds in Aromatic Esters with CO2: Understanding Remarkable Ligand and Traceless‐Directing‐Group Effects

The mechanism of the Ni⁰‐catalyzed reductive carboxylation reaction of C(sp²)?O and C(sp³)?O bonds in aromatic esters with CO₂ to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C?O bond cleavage, reductive elimination, and/or CO₂ insertion. Of these steps, C?O bond cleavage was found to be rate‐determining, and it occurred through either oxidative addition to form a Ni^II intermediate, or a radical pathway that involved a bimetallic species to generate two Ni^I species through homolytic dissociation of the C?O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp²)?O and C(sp³)?O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni⁰ catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two Ni^I species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C?O bonds were also employed to investigate several experimentally observed phenomena, including ligand‐dependent reactivity and site‐selectivity.     

N-Monobromamide: Darstellung durch Bromierung mit „Dibromisocyanursäure”, Eigenschaften

By reaction of primary carboxamides with “dibromoisocyanuric acid” (DBI) N-monobromoamides can be readily obtained as well as the N,N-dibromoamides described in an earlier paper¹. Reactions, some of them new, and properties of these compounds are described and compared with those of the N,N-dibromoamides. Like other compounds bearing the NHBr group^{2, 3} the N-monobromocarboxamides disproportionate at room temperature according to: 2 RCONHBr ? ? RCONH₂+RCONBr₂. For CH₃CONHBr the equilibrium constant was found to beK=0.02. In aqueous solution they behave as weak acids. The dissociation constants of eight compounds [R=?CH₃, ?C₂H₅, ?CH₂Cl, ?CHCl₂, ?CCl₃, ?CF₃, ?C(CH₃)₃ and ?C₆H₅] were measured: they differ from those of the corresponding carboxylic acids by about three powers of ten.     

The influence of the negative hyperconjugation is relevant for the analysis of the π-π<Superscript>*</Superscript> conjugation with the mono-substitution and di-substitution of H<Subscript>2</Subscript>C= by O= and/or HN= in <Emphasis Type="Italic">trans</Emphasis>-buta-1,3-diene?

We have studied prop-2-en-1-imine (1), prop-2-enal (2), ethane-1,2-diimine (3), ethanedial (4), and 2-iminoacetaldehyde (5) to investigate the influence of the negative hyperconjugation in π-π^* interaction with the substitution of =CH₂ by =NH and/or =O in trans-buta-1,3-diene (6). The analyzes of the π-π^* interaction performed from evaluation of the π molecular orbital diagrams and electron localization function method demonstrated, that compared to 6, the substituted compounds 1-5 presented lower electron conjugation, especially in the structures bearing =O. The geometric parameters, natural bond orders, and topological analysis realized by quantum theory of atoms in molecules method indicated a predominant C-C and C=C character for the simple and double C-C bonds in the substituted compounds, 1-5, as compared to 6. Compound 4 had the highest enthalpy of formation, which reflected the lowest π-π^* interaction, maintained by the two =O conjugated groups. The natural bond orbital (NBO) and natural resonance theory (NRT) methods revealed that the π-π^* electron delocalization in substituted compounds, 1-5, is lower than in 6 from, firstly, of the less favorable interactions: π(X=C) ? π^*(C=C) and π(X=C) ? π^*(C=X), despite of the larger π(C=C) ? π^*(C=X) conjugation, with X = N and/or O, of 1-5 than π(C=C) ? π^*(C=C) of 6. But, most importantly, the weight of the interaction: n_π(O) ? σ^*(C-C), was determined from NBO and NRT methods as proportional to the π-π^* conjugation and thus demonstrating be decisive to establish the level of π electronic delocalization.     

Nmr study of the VixSiX4−x compounds with X = Cl,Br; x = 0, 1, 2, 3

¹H, ¹³C and ²⁹Si NMR data for the compounds Vi_xSiX_4?x are reported. While the ¹H and ¹³C resonances from the π system are indicative of the electron-withdrawing inductive effect (-I) of the halogens, the ²⁹Si chemical shift data reveal not only a shift contribution originating from this inductive effect but also the important influence of a {d, σ^*-π} hyperconjugation [1]. This back-donation originates from the vinyl π system and not from the halogens. The chemical shift data and the coupling constants also show an important influence from steric interaction and even from an electric field effect caused by polarization of the silicon—halogen bond.     

Construction and Electrochemical Property Studies of DNA Duplexes Tethered to Gold Electrode via Au−C Bond

Effective linkage of DNA onto metal surfaces plays a crucial role in the applications of DNA as electrochemical recognition, signal output and amplification devices for gene and protein detections, specific analyte recognitions, catalysis, and so on. Here we report a promising and operationally simple approach for the construction of double‐stranded DNA‐linked Au interface via Au?C bond (RdsDNA‐C?Au), upon efficient in situ cleavage of trimethylsilyl end group of 4‐[(trimethylsilyl) ethynyl] benzoic acid and subsequent dehydration condensation between NH₂‐modified DNA and benzoic acid. Due to the introduction of large conjugated π group (4‐carboxyphenylethynyl) as the “bridge bond”, the conductivity of the RdsDNA‐C?Au interface is greatly improved. As a result, under commonly used DNA packing density (>0.5 pmol cm^?2), the surface‐confined electron transfer at the interface is simply mediated by the stacked‐bases of dsDNA, independent of the orientation of dsDNA (tethered to the electrode at 5′‐ or 3′‐end). Also, compared to the traditional RdsDNA‐S?Au interface via alkanethiol linker, the RdsDNA‐C?Au interface displays more sensitive electrochemical response and excellent stability. Due to the better stability, conductivity and simple electron transfer mechanism, the RdsDNA‐C?Au interface is anticipated to be widely used in electrochemistry‐involved molecular recognitions, gene and protein detections with higher sensitivity and accuracy.